A synthetic aperture radar (SAR) system comprises a radar signal transmitter and receiver operating on a movable platform, such as an aeroplane or satellite; and a remote processing station connected over a radio channel to the movable platform. The transmitter sends a radar signal into a monitored ground or sea area, and the receiver receives the radar echo reflected by the area, and transmits it to the remote processing station, which processes the radar echo to obtain a two-dimensional map of the monitored area.
The transmitted radar signal comprises a succession of microwave-band electromagnetic pulses modulated by linear frequency, or so-called CHIRP, modulation and transmitted at regular time intervals. And the processing station coherently combines the radar echoes corresponding to the transmitted pulses to obtain high-azimuth-resolution maps of extensive areas using relatively small transmitting antennas. CHIRP pulse modulation, on the other hand, provides for achieving high resolution perpendicular to the azimuth direction.
Exceptionally large areas may be covered using a number of satellites or so-called satellite constellation.
Fixed reference points on the maps are located using transponders or so-called “corner reflectors”, which are located at the fixed reference points to receive the radar signal and retransmit it transparently, after simply amplifying it. The retransmitted radar signal is usually stronger than the signals reflected by the surrounding environment and constituting the radar echo, so the fixed reference points appear brighter on the constructed map.
The above method, however, is not very effective in identifying a number of fixed and/or moving targets, e.g. ships, vehicles, or people, within the monitored area. That is, though positions (brighter points) can always be detected on the map, the targets corresponding to the detected positions cannot be identified accurately, especially in the case of fast-moving targets.